Method and apparatus for improving froth flotation

ABSTRACT

An apparatus for supplying a reagent to a froth flotation cell. The flotation cell is fed by a flotation gas feed line. A predetermined volume is in fluid communication with said flotation gas feed line. The volume has a gas inlet on an upstream side, a gas outlet on a downstream side. An atomiser is positioned intermediate the inlet and outlet to atomise the reagent into the predetermined volume. The atomised reagent is then entrained with the flotation gas entering said cell.

TECHNICAL FIELD

The present invention relates to froth flotation and in particularmethods and apparatus for maximising flotation recovery and yield whileoptimising reagent usage.

BACKGROUND ART

Separation of fine coal from ash by flotation is based on the differencein wettability (or hydrophobicity) between the coal and ash. Coal isnaturally hydrophobic (fear of water), while ash is naturallyhydrophilic (love for water). In flotation, air is introduced into thecoal-ash slurry. The hydrophobic coal particles cling to the air bubblesand rise with them to the top of the flotation cell where they arecollected as concentrate, whereas the hydrophilic ash particles sink tothe bottom of the cell and report to tailings. Thus the fine coal andash particles are separated.

If no frother was added to the flotation process the air bubbles wouldnot be stable, would tend to coalesce and break up and any coalparticles adhering to them would sink back into the pulp. By theaddition of certain surface-active organic compounds, called frothers, astable froth is formed on the surface which facilitates transfer of thefloated coal particles from the cell to the collection launders.

Current practice in all flotation applications is to add the frother tothe liquid (slurry) phase and allow it to diffuse from the slurry to theair liquid interface. This method of addition, however, can beinefficient due to inadequate frother dispersion within the slurry andthe requirement for frother migration within the liquid phase. Incurrent coal flotation plants, frother quantities in the order of 5 to20 ppm (parts per million) are added (ie, 5 to 20 grams of frother into1 million grams of fresh coal slurry). At such low dosage rates it isdifficult to achieve uniform dispersion of the frother within theslurry. Also important is that the frother is required to act on theair-liquid interface. Frother added to the slurry is therefore requiredto migrate from the liquid phase to the air-liquid interface when aircomes into contact with slurry.

Frother is a very important operating parameter in Jameson Cells and hasa major impact on fine coal yields from flotation. The Jameson Cell andits operation is discussed in detail in Australian Patent No 677452(which is incorporated herein by reference). In addition to creating astable frother layer on the cell surface, frother significantly improvesthe air vacuum and hence air flow rate. Higher airflow rates generatefiner and more numerous air bubbles and higher bubble rise velocities.Finer and larger quantities of air bubbles mean there is more airsurface area for the fine coal particles to be attached. This coupledwith higher air bubble rise velocities, results in much higher coalyields from flotation.

If frother is added to the liquid phase, as per current practice, thento achieve optimum mass yields from the flotation circuit 20 ppm offrother is recommended. However, in reality most sites are only able toadd 5 to 10 ppm. This is a consequence of the design of coal preparationplants and higher levels of frother are not achievable without expendingconsiderable capital to change the plant design, in particular the waterbalance. At most coal preparation plants the tailings from the flotationcircuit reports to the thickener. The overflow from the thickener isprocess water that is recirculated back to the plant, including thecoarse coal circuit. When frother levels of greater than 5 to 10 ppm areadded to the flotation circuit, due to the inefficiencies of addingfrother to the liquid phase, residual frother reports to the tailingsand hence process water. This creates major operational upsets in thecoarse coal circuit (“frothing out the plant”) and therefore frotherdosages have to be limited.

In addition, various reagents are used to assist in recovery of otherminerals such as valuable sulphide or secondary minerals. There is aneed to increase the effectiveness of various reagents used in frothflotation such as collectors and frothers and thus improve the recoveryof valuable minerals using known reagents.

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

It is an object of the present invention to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative.

DISCLOSURE OF INVENTION

In a first aspect, the present invention provides an apparatus forsupplying a reagent to a froth flotation cell, said apparatus comprisinga flotation gas feed line and a predetermined volume in fluidcommunication with said flotation gas feed line, said volume having agas inlet on an upstream side, a gas outlet on a downstream side and anatomiser positioned intermediate the inlet and outlet, said atomiserbeing adapted to atomise said reagent such that said atomised reagent isentrained with flotation gas entering said cell.

The predetermined volume may be formed within the flotation gas feedline or, alternatively, the volume may take the form of a chamber influid communication with the flotation line. This second option isparticularly suitable for retrofitting of the apparatus to flotationcells, which of course, already have a flotation gas feed line.

If a froth flotation cell was being constructed with the aforementionedapparatus from scratch, for example, the atomiser of course may bepositioned anywhere on the flotation gas feed line. A particularlysuitable embodiment for use with Jameson Cells is the incorporation ofthe atomiser in the air distributor which feeds air to the variousdowncomers in the Jameson Cell.

The apparatus is suitable for use on a flotation gas feed line which issub-atmospheric, for instance, where the cell is a Jameson Cell, orwhere the flotation gas feed line is at or greater than atmosphericpressure.

Where the atomiser is positioned within the chamber on the gas feedline, it is preferable that the atomiser is positioned adjacent theinlet of that chamber and spaced a sufficient distance from the outletto minimise impact and condensation of the atomised reagent on thechamber wall.

To further reduce condensation of the reagent, the chamber and/orflotation gas feed line between the volume and the cell may be thermallyinsulated.

Generally, the dimensions of the chamber will depend upon a number offactors including flotation slurry feed rates, flotation gas feed rates,the type and amount of reagent to be atomised, etc. In one embodiment,the dimensions of the chamber are calculated by determining anatomisation area from said atomiser, ie the area covered by the sprayemanating from the atomiser. An appropriate clearance, eg 200 mm maythen be added to this figure to avoid direct impact of the reagent mistemanating from the atomiser onto the walls of the chamber.

In most installations it is envisaged that each flotation cell wouldhave a defined volume/atomiser in the flotation gas line.

It will be understood by persons skilled in the art that the atomisercan be any suitable apparatus for atomising a liquid reagent such asnozzles, jet sprays, ultrasonic generators, etc.

In a second broad aspect, the present invention provides a method ofsupplying a reagent to a froth flotation cell comprising defining on aflotation gas inlet side to the cell, a predetermined volume having agas inlet and a gas outlet, positioning within said volume an atomiserto produce an atomised reagent within said volume, and passing flotationgas through said volume such that said atomised reagent is entrainedwith a flotation gas entering the flotation cell.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words ‘comprise’, ‘comprising’, and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to”.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example only, withreference to the accompanying embodiments exemplified in the drawings asfollows:

FIG. 1 is a front elevational view of a chamber to be used inconjunction with a flotation cell in accordance with a first embodimentof the present invention,

FIG. 2 is an end elevational view of the interior of the chamber of FIG.1, and

FIG. 3 is a schematic elevational view of the chamber in use with aJameson Cell.

FIGS. 4 to 6 are graphs of test results for % ash in tails, % yield and% combustibles recovery respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

In the embodiments shown in FIGS. 1 to 3, the predetermined volume influid communication with the flotation gas feed line is provided by achamber 10. It will be understood by persons skilled in the art,however, that a separate chamber 10 is not required and the inventionmay be embodied by any predetermined volume formed on or in fluidcommunication with flotation gas feed line 100.

In particular, the chamber 10 is shown on the flotation gas feed line100 of Jameson Cell. The flotation gas enters the cell through flotationgas feed line 100 into air distributor 150 and from the distributor viaconnector 160 to a downcomer 170.

The flotation slurry is fed to the downcomer 100 by means of slurrydistributor 200 and slurry feed line 210.

The embodiment shown in FIGS. 1 to 3 wherein the predetermined volume asprovided by chamber 10 is particularly suitable for retrofitapplications. As will be clear to persons skilled in the art, to includechamber 10 on a flotation gas feed line is a reasonably simple process.

For a purpose built facility, however, the predetermined volume for theatomiser 60 can be positioned anywhere on the flotation gas feed line.In one particular embodiment it is envisaged that the atomising means 60may be provided in the air distributor 150. In such an instance, the airdistributor has the dual roles of distributing flotation gas to thedowncomers and as the predetermined volume for atomisation of theflotation reagent.

Referring now to FIGS. 1 and 2, the chamber 10 comprises an upstreamwall 20, downstream wall 40 on which are positioned inlet and outletpipe connectors 25 and 45 which, as discussed below, are adapted to beconnected to a flotation gas feed pipe providing gas to the flotationcell.

On upstream wall 20 is positioned atomising means 60, in this case, aplurality of nozzles 65. The upstream wall 20 may be provided with aseries of viewing windows 26 to view operation of the atomising means 60as will be discussed below. A drainage hole 70 may also be provided toallow for removal of condensed reagent as will be discussed below.

As can be seen in FIGS. 1 and 2, the atomising means 60 is provided byan annular array of nozzles 65 around inlet 25. While this is notessential to the invention, it has been found that such an array ofnozzles provides for good atomisation and entrainment of the reagentmist with the flotation gas entering the chamber.

In this embodiment, inlet 25 and outlet 45 are essentially coaxial withthe chamber 20. While not essential, this is also preferred since itpermits for rapid flow of inlet air through the chamber with theentrained reagent. As will be clear to persons skilled in the art, anyoffset of inlet 25 to outlet 45 may interrupt the smooth flow throughthe chamber and create unnecessary turbulence or eddies therein reducingentrainment of the reagent mist with the flotation gas entering the celland promote condensation on the chamber walls.

Turning now to FIG. 3, the operation of the chamber will now bediscussed.

FIG. 3 shows the chamber 10 positioned on the gas inlet line 100 feedinga Jameson Cell 200. The apparatus is suitable for other flotationapparatus but for the sake of simplicity will be discussed here withreference to a Jameson Cell.

The gas inlet line 100 contains a valve 120 which constricts gas line100 thereby controlling the partial vacuum in the Jameson Cell,controlling the speed and quantity of gas, in this case air, whichenters the Jameson Cell 200. Details of the Jameson Cell can be found ina number of patents/applications including Australian Patent No 677542(which is incorporated herein by reference).

In use, atomising means 60 is connected to a particular reagent. Theembodiment described will relate to atomised addition of frother,however, it will be understood that other reagents can be atomised in asimilar fashion.

The nozzles 65 are supplied with compressed gas such as air and frother.The frother is pumped to the nozzle at a metered rate and compressed airis supplied under pressure. Inside the nozzle, the compressed airimpacts with the frother breaking it up into small droplets and forcingit out of the nozzle as an aerosol, spray or mist.

The nozzles provide a spray of reagent which is entrained with the airpassing through the chamber 10 into the cell. In the embodiment shown,the nozzle spray is essentially parallel with the air stream through thechamber. In other embodiments, the nozzles may be adjustable such thatthe spray from the nozzles converge, diverge or extend substantiallyparallel. As mentioned above, it is preferred that turbulence andresidence time in the chamber is reduced by providing a fast smoothentry and exit into and out of the chamber. In this regard it will benoted that exit wall 40 is tapered to provide such a smooth exit. TheApplicant has found that at conventional frother dosages, the use of theinventive method and apparatus substantially improves yield and recoveryin the flotation cell.

The four windows 26 mounted on wall 20 allow for visual inspection ofthe mist created by the nozzles. This permits monitoring of the spraypattern as well as noting changes in reagent character or consumptionand help identify blocked or non-operational nozzles. It also allows forexperimentation with different spray patterns, nozzle air pressures etcto determine their effect on nozzle performance.

Preferably, wall 20 is flanged such that it allows for easy removal andaccess to the nozzles either as a group or individually.

One of the major difficulties with atomising of reagents for subsequentfeeding to the flotation cell is condensation of the spray or mist,either on the walls of the chamber or in the gas line 100 downstream ofchamber 10.

There are a number of factors which influence the condensation rateincluding the size of the droplets being issued from the nozzles,contact of droplets with surfaces, residence time in the chamber andcontact with surfaces of different temperatures.

Unlike many conventional aerosol systems, which require heating of theaerosol fluid, the nozzles or the aerosol chamber, the present apparatusand method provides excellent control of condensation of the aerosolwithout the need for such expensive or complex heating systems.

In this regard, the present invention provides for modification ofseveral operational parameters to reduce condensation of the reagentspray or mist. Firstly, it has been found that the nozzles operate bestwith relatively low reagent flow, relative to the compressed gas beingfed to then nozzle. It appears that low flow of the liquid reagenttogether with high air pressure results in a mist of finer droplet size.

Another parameter is the distance between walls 20 and 40. As will beclear to persons skilled in the art, if wall 40 is placed too close towall 20, the droplets issuing from nozzle 65 will impact wall 40 andcondense thereon. Accordingly, the distance between walls 20 and 40should be adjusted to ensure minimal condensation arising from contactof the mist or spray on wall 40.

Another step to reduce condensation is to maximise airflow through thechamber. This is performed in the embodiment shown by incorporating thechamber as a feature of the air inlet line on the Jameson Cell, in otherwords, all air entering the Jameson Cell has to pass through thechamber, ie maximum air flow and air speed.

There are of course significant advantages, apart from reducedcondensation, arising from passing all inlet flotation gas through thechamber. These include better mixing, greater distribution of thefrother in the pulp and reduction in expenses since additional pipingand/or pressurising systems are not required to force the mist into thecell.

Another way of reducing condensation is to insulate the chamber anddownstream pipe work to minimise temperature differences betweenconditions within the chamber and the chamber wall. While it is not yetproved, the applicant believes one of two things will happen to largerdroplets within the chamber. They will either be impacted by air passingthrough the chamber and reduced inside or they will contact the surface,condense and be collected for recycling via drainage port 70. Smallerdroplets will be entrained in the inlet air through the air distributorto the Jameson Cell downcomer.

It will be appreciated that such an arrangement is also extremelyflexible and less subject to environmental influences than theaforementioned conventional systems.

The Applicants have indeed found that the apparatus and method operatessuccessfully in quite different environments e.g. high temperature orhumidity as well as low temperature or dry environments. Suchflexibility appears absent from prior art devices which rely onextensive temperature control systems to remain within suitableoperational parameters.

EXAMPLE 1

Test work has been conducted at Sunwater Laboratories, Rocklea, Brisbaneusing two chambers of different dimensions with 3 nozzles. The resultsof this testing is discussed below.

Two chambers were tested with various nozzle flows and airflow. Thenozzles were supplied with MIBC frother. Flow through the nozzledepended upon the MIBC pump dosage rate. The compressed air requirementfor 3 nozzles at 300 kPa was 5 m³/hour. The compressed air to thenozzles was dry and filtered so as to reduce blockage of the nozzles.The results are shown in Table 1. TABLE 1 Sunwater Laboratories TestData No. DC's = 24 m³ per Pulp flow = 70 DC Air/Pulp Ratio = 0.8 No.Nozzles = 14 MIBC MIBC Air Flow (not MIBC total Flow Air MIBC Thru incl.(incl. (incl. Chamber Nozzle Pressure Pump Flow Chamber Condensationlosses) losses) losses) Size Size KPa Speed % 1/hr m³/hr Losses % ppmppm 1/hr 800 × 800 1650 294 25 2.05 71 7% 7 7 9.6 800 × 800 1650 294 504.09 72 22% 14 11 19.1 800 × 800 1650 294 100 8.69 71 33% 30 20 40.6 400× 400 1650 294 25 2.03 76 6% 7 7 9.5 400 × 400 1650 294 50 4.13 73 18%14 12 19.3 400 × 400 1650 294 100 8.83 74 39% 31 19 41.2

As can be seen from Table 1, with both chamber sizes, lower flows to thenozzles resulted in reduced condensation of the MIBC frother mist andtherefore reduced wastage of the MIBC frother. In this regard, it isbelieved that a significant advantage arises from the present inventionin that the reagent, in this case the froth acting agent, is provideddirectly into the column of froth formed in the Jameson Cell downcomerrather than the pulp. This clearly has a significant advantage over theprior art in that the frother is provided to the most efficient locationfor its use, ie the point at which froth generation takes place.

In the example where pump speed was 100%, ie maximum flow to the nozzlesat least a third of the frother was lost to condensation. This condensedfrother may be retrieved, however, via drainage line 70 and recycledback to the system. Further, it is preferred that air distributor 150have a sloping floor which allows any reagent/frother condenseddownstream of chamber to drain to a single point for recycling back tothe Jameson Cell.

EXAMPLE 2

This example was carried out at Oaky Creek J5000/24 Coal Prep Plant, acomparison was conducted on a Jameson Cell using the aforementionedmethod and apparatus to atomise the frother as compared withconventional addition of frother to the pulp.

Table 2 below shows the results of percentage ash in the tails,percentage yield and percentage combustibles recovered from the coalundergoing flotation. TABLE 2 Comparison of Chamber/Atomiser withConventional Frother Dosage Tails Yield Comb. Rec. OFF ON OFF ON OFF ON25.6 54.2 53.1 72.4 59.2 84.1 33.5 54.4 39.7 76.5 45.5 86.9 45.0 57.963.3 72.4 74.5 85.2 45.9 52.7 54.8 59.3 67.8 74.2 38.8 55.0 46.9 58.657.5 74.5Note,ON = Use of Chamber/AtomiserOFF = Conventional Frother Addition

The various samples were dosed with 5 ppm frother (MIBC), slurry rate of1560 m³/hr

In every case, use of the chamber 10 to atomise and add frother provideda substantial increase over conventional mechanisms. A graphicalrepresentation of the results of Table 2 are shown in FIGS. 4 to 6.

As discussed above, while the embodiment shown is in regard to a JamesonCell, which uses an air inlet line below atmospheric pressure, it willbe understood that it is also suitable for use with other flotationgases and flotation cells with pressurised flotation gas inlets.

Testing conducted by the Applicants has shown remarkable results todate. For instance, current MIBC consumption at the Oaky Creek site isless than 6 ppm. A concentration above this limit would adversely affectthe remainder of the circuit. However, 6 ppm MIBC is well below therecommended 20 ppm for optimum Jameson Cell operation when MIBC is addedas a liquid.

Test work has indicated that aerosol/mist addition of MIBC may reducethe quantity of MIBC required for optimum Jameson Cell operation by atleast 75%. Hence, MIBC consumption using the inventive method andapparatus will range from between 4 to 7 ppm. At these levels and asevidenced by the attached data, an increased coal yield of at least 5%will clearly provide substantial additional revenue in terms ofrecovered product, but also substantial savings in terms of MIBCconsumption.

In addition, using the inventive method and apparatus increasesefficiency of the Jameson cell at conventional dosage levels, eg around5-10 ppm

It will be understood by persons skilled in the art that the abovementioned described method and apparatus may be embodied in other formswithout departing from the spirit or scope of the present invention.

1. An apparatus for supplying a reagent to a froth flotation cell, saidapparatus comprising a flotation gas feed line and a predeterminedvolume in fluid communication with said flotation gas feed line, saidvolume having a gas inlet on an upstream side, a gas outlet on adownstream side and an atomiser positioned intermediate the inlet andoutlet, said atomiser being adapted to atomise said reagent into saidpredetermined volume such that said atomised reagent is entrained withflotation gas entering said cell.
 2. An apparatus as claimed in claim 1,wherein the volume is formed within the flotation gas inlet line.
 3. Anapparatus as claimed in claim 1, wherein the volume is formed by achamber in fluid communication on the flotation gas feed line.
 4. Anapparatus as claimed in claim 1, wherein said flotation gas feed line issub-atmospheric.
 5. An apparatus as claimed in claim 1, wherein theflotation cell is a Jameson Cell.
 6. An apparatus as claimed in claim 5,wherein said atomiser is located adjacent an air distributor in thedowncomer of the Jameson Cell.
 7. An apparatus as claimed in claim 1,wherein said flotation gas feed line is at or greater than atmosphericpressure.
 8. An apparatus as claimed in claim 3, wherein the atomiser ispositioned adjacent the inlet of the chamber.
 9. An apparatus as claimedin claim 3, wherein the atomiser is spaced a sufficient distance fromthe outlet of the chamber to minimise impact and condensation of theatomised reagent on the chamber wall.
 10. An apparatus as claimed inclaim 1, wherein the volume and/or flotation gas feed line between thevolume and cell are thermally insulated.
 11. An apparatus as claimed inclaim 1 wherein the atomiser is adapted to atomise a frother into saidpredetermined volume.
 12. A method of supplying a reagent to a frothflotation cell comprising defining on a flotation gas inlet side to thecell, a predetermined volume having a gas inlet and a gas outlet andpositioning within said volume an atomiser to produce an atomisedreagent within said volume, and passing flotation gas through saidvolume such that said atomised reagent is entrained with a flotation gasentering the flotation cell.
 13. A method as claimed in claim 12,wherein the predetermined volume is provided in the flotation gas inletline.
 14. A method as claimed in claim 12, wherein the predeterminedvolume is provided by a chamber located on the flotation gas feed line,the atomiser producing an atomised reagent into said chamber forentrainment with a flotation gas passing therethrough.
 15. A method asclaimed in claim 12, wherein the flotation gas feed line is run atsub-atmospheric pressure.
 16. A method as claimed in claim 12, whereinthe flotation gas feed line is run at or greater than atmosphericpressure.
 17. A method as claimed in claim 12, wherein the predeterminedvolume is defined by measuring a projected atomisation volume of theatomiser, adding an appropriate clearance thereto and ensuring that saidpredetermined volume is equal to or greater than the resultantdimensions.
 18. A method as claimed in claim 1, wherein the reagent is afrother.
 19. An apparatus for supplying a reagent to a froth flotationcell substantially as herein described with reference to any one of theembodiments of the invention illustrated in the accompanying drawingsand/or examples.
 20. A method for supplying a reagent to a frothflotation cell substantially as herein described with reference to anyone of the embodiments of the invention illustrated in the accompanyingdrawings and/or examples.